socket(2) — Linux manual page


socket(2)                  System Calls Manual                 socket(2)

NAME         top

       socket - create an endpoint for communication

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <sys/socket.h>

       int socket(int domain, int type, int protocol);

DESCRIPTION         top

       socket() creates an endpoint for communication and returns a file
       descriptor that refers to that endpoint.  The file descriptor
       returned by a successful call will be the lowest-numbered file
       descriptor not currently open for the process.

       The domain argument specifies a communication domain; this
       selects the protocol family which will be used for communication.
       These families are defined in <sys/socket.h>.  The formats
       currently understood by the Linux kernel include:
       Name         Purpose                                    Man page
       AF_UNIX      Local communication                        unix(7)
       AF_LOCAL     Synonym for AF_UNIX
       AF_INET      IPv4 Internet protocols                    ip(7)
       AF_AX25      Amateur radio AX.25 protocol               ax25(4)
       AF_IPX       IPX - Novell protocols
       AF_APPLETALK AppleTalk                                  ddp(7)
       AF_X25       ITU-T X.25 / ISO-8208 protocol             x25(7)
       AF_INET6     IPv6 Internet protocols                    ipv6(7)
       AF_DECnet    DECet protocol sockets
       AF_KEY       Key management protocol, originally
                    developed for usage with IPsec
       AF_NETLINK   Kernel user interface device               netlink(7)
       AF_PACKET    Low-level packet interface                 packet(7)
       AF_RDS       Reliable Datagram Sockets (RDS) protocol   rds(7)
       AF_PPPOX     Generic PPP transport layer, for setting
                    up L2 tunnels (L2TP and PPPoE)
       AF_LLC       Logical link control (IEEE 802.2 LLC)
       AF_IB        InfiniBand native addressing
       AF_MPLS      Multiprotocol Label Switching
       AF_CAN       Controller Area Network automotive bus
       AF_TIPC      TIPC, "cluster domain sockets" protocol
       AF_BLUETOOTH Bluetooth low-level socket protocol
       AF_ALG       Interface to kernel crypto API
       AF_VSOCK     VSOCK (originally "VMWare VSockets")       vsock(7)
                    protocol for hypervisor-guest
       AF_KCM       KCM (kernel connection multiplexer)
       AF_XDP       XDP (express data path) interface

       Further details of the above address families, as well as
       information on several other address families, can be found in

       The socket has the indicated type, which specifies the
       communication semantics.  Currently defined types are:

              Provides sequenced, reliable, two-way, connection-based
              byte streams.  An out-of-band data transmission mechanism
              may be supported.

              Supports datagrams (connectionless, unreliable messages of
              a fixed maximum length).

              Provides a sequenced, reliable, two-way connection-based
              data transmission path for datagrams of fixed maximum
              length; a consumer is required to read an entire packet
              with each input system call.

              Provides raw network protocol access.

              Provides a reliable datagram layer that does not guarantee

              Obsolete and should not be used in new programs; see

       Some socket types may not be implemented by all protocol

       Since Linux 2.6.27, the type argument serves a second purpose: in
       addition to specifying a socket type, it may include the bitwise
       OR of any of the following values, to modify the behavior of

              Set the O_NONBLOCK file status flag on the open file
              description (see open(2)) referred to by the new file
              descriptor.  Using this flag saves extra calls to fcntl(2)
              to achieve the same result.

              Set the close-on-exec (FD_CLOEXEC) flag on the new file
              descriptor.  See the description of the O_CLOEXEC flag in
              open(2) for reasons why this may be useful.

       The protocol specifies a particular protocol to be used with the
       socket.  Normally only a single protocol exists to support a
       particular socket type within a given protocol family, in which
       case protocol can be specified as 0.  However, it is possible
       that many protocols may exist, in which case a particular
       protocol must be specified in this manner.  The protocol number
       to use is specific to the “communication domain” in which
       communication is to take place; see protocols(5).  See
       getprotoent(3) on how to map protocol name strings to protocol

       Sockets of type SOCK_STREAM are full-duplex byte streams.  They
       do not preserve record boundaries.  A stream socket must be in a
       connected state before any data may be sent or received on it.  A
       connection to another socket is created with a connect(2) call.
       Once connected, data may be transferred using read(2) and
       write(2) calls or some variant of the send(2) and recv(2) calls.
       When a session has been completed a close(2) may be performed.
       Out-of-band data may also be transmitted as described in send(2)
       and received as described in recv(2).

       The communications protocols which implement a SOCK_STREAM ensure
       that data is not lost or duplicated.  If a piece of data for
       which the peer protocol has buffer space cannot be successfully
       transmitted within a reasonable length of time, then the
       connection is considered to be dead.  When SO_KEEPALIVE is
       enabled on the socket the protocol checks in a protocol-specific
       manner if the other end is still alive.  A SIGPIPE signal is
       raised if a process sends or receives on a broken stream; this
       causes naive processes, which do not handle the signal, to exit.
       SOCK_SEQPACKET sockets employ the same system calls as
       SOCK_STREAM sockets.  The only difference is that read(2) calls
       will return only the amount of data requested, and any data
       remaining in the arriving packet will be discarded.  Also all
       message boundaries in incoming datagrams are preserved.

       SOCK_DGRAM and SOCK_RAW sockets allow sending of datagrams to
       correspondents named in sendto(2) calls.  Datagrams are generally
       received with recvfrom(2), which returns the next datagram along
       with the address of its sender.

       SOCK_PACKET is an obsolete socket type to receive raw packets
       directly from the device driver.  Use packet(7) instead.

       An fcntl(2) F_SETOWN operation can be used to specify a process
       or process group to receive a SIGURG signal when the out-of-band
       data arrives or SIGPIPE signal when a SOCK_STREAM connection
       breaks unexpectedly.  This operation may also be used to set the
       process or process group that receives the I/O and asynchronous
       notification of I/O events via SIGIO.  Using F_SETOWN is
       equivalent to an ioctl(2) call with the FIOSETOWN or SIOCSPGRP

       When the network signals an error condition to the protocol
       module (e.g., using an ICMP message for IP) the pending error
       flag is set for the socket.  The next operation on this socket
       will return the error code of the pending error.  For some
       protocols it is possible to enable a per-socket error queue to
       retrieve detailed information about the error; see IP_RECVERR in

       The operation of sockets is controlled by socket level options.
       These options are defined in <sys/socket.h>.  The functions
       setsockopt(2) and getsockopt(2) are used to set and get options.

RETURN VALUE         top

       On success, a file descriptor for the new socket is returned.  On
       error, -1 is returned, and errno is set to indicate the error.

ERRORS         top

       EACCES Permission to create a socket of the specified type and/or
              protocol is denied.

              The implementation does not support the specified address

       EINVAL Unknown protocol, or protocol family not available.

       EINVAL Invalid flags in type.

       EMFILE The per-process limit on the number of open file
              descriptors has been reached.

       ENFILE The system-wide limit on the total number of open files
              has been reached.

              Insufficient memory is available.  The socket cannot be
              created until sufficient resources are freed.

              The protocol type or the specified protocol is not
              supported within this domain.

       Other errors may be generated by the underlying protocol modules.

STANDARDS         top


       SOCK_NONBLOCK and SOCK_CLOEXEC are Linux-specific.

HISTORY         top

       POSIX.1-2001, 4.4BSD.

       socket() appeared in 4.2BSD.  It is generally portable to/from
       non-BSD systems supporting clones of the BSD socket layer
       (including System V variants).

       The manifest constants used under 4.x BSD for protocol families
       are PF_UNIX, PF_INET, and so on, while AF_UNIX, AF_INET, and so
       on are used for address families.  However, already the BSD man
       page promises: "The protocol family generally is the same as the
       address family", and subsequent standards use AF_* everywhere.

EXAMPLES         top

       An example of the use of socket() is shown in getaddrinfo(3).

SEE ALSO         top

       accept(2), bind(2), close(2), connect(2), fcntl(2),
       getpeername(2), getsockname(2), getsockopt(2), ioctl(2),
       listen(2), read(2), recv(2), select(2), send(2), shutdown(2),
       socketpair(2), write(2), getprotoent(3), address_families(7),
       ip(7), socket(7), tcp(7), udp(7), unix(7)

       “An Introductory 4.3BSD Interprocess Communication Tutorial” and
       “BSD Interprocess Communication Tutorial”, reprinted in UNIX
       Programmer's Supplementary Documents Volume 1.

Linux man-pages (unreleased)     (date)                        socket(2)

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